Bilanchuk V. Influence of isomorphic substitution on electrical and optical properties of solid solutions based on Cu7BX5I (B= Ge, Si; X=S, Se)

The thesis is devoted to the studies of influence of isomorphic substitution on structural, electrical and optical properties of Cu7BX5I (B= Ge, Si; X=S, Se) crystals and solid solutions on their base. Cu7Ge(S1-xSex)5I and Cu7(Ge1-xSix)S5I solid solutions systems are shown to be continuous with a linear increase of the cubic lattice parameter at anion S Se substitution and cation Si Ge substitution according to the Vegard law. For Cu7GeSе5I, Cu7GeS5I, Cu7SiS5I, Cu7Ge(S1-xSex)5I, and Cu7(Ge1-xSix)S5I crystals a linear increase of electrical conductivity with temperature is observed, indicating the thermoactivation character of the conductivity. Anionic (S Se) and cationic (Si Ge) substitution leads to a nonlinear increase of the electrical conductivity by more than an order of magnitude. In superionic ceramics based on nanoscale Cu7GeS5I, a high value of the electrical conductivity is related to an increase of ion diffusion in the intercrystallite space with the particle size decrease. For electrodes prepared from the Cu7Ge(S1-xSex)5I and Cu7(Ge1-xSix)S5I solid solution crystals, the current-voltage diagrams are shown to possess one anodic and two cathodic maxima, decreasing with sulphur and germanium content in the corresponding solid solutions. The areas of electrochemical stability for Cu7Ge(S1-xSex)5I and Cu7(Ge1-xSix)S5I solid solution crystals are determined. With the substitution of S by Se atoms as well as Si by Ge atoms in Cu7Ge(S1-xSex)5I end Cu7(Ge1-xSix)S5I solid solution crystals the electrochemical stability ranges shrink, which is explained by the decrease of the chemical bond ionic component and the increase of the covalent-and-metallic component. The Urbach behaviour of the optical absorption edge of the crystals under investigation is revealed, being determined by a strong electron-phonon interaction. In the solid solution crystals the optical absorption edge is strongly affected by compositional disordering of the crystal lattice while the electron-phonon interaction is enhanced. The temperature dependences of the Urbach absorption edge parameters, namely the optical pseudogap and the Urbach energy, are described in the framework of the Einstein model for Cu7GeSe5I and Cu7SiS5I crystals as well as for Cu7Ge(S1-xSex)5I and Cu7(Ge1-xSix)S5I solid solutions in the temperature interval of 77-300 K. At anionic (S Se) and cationic (Si Ge) substitution, a nonlinear decrease of the optical pseudogap is observed as well as the Urbach energy behaviour characteristic for solid solutions. The compositional dependences of the Urbach energy in Cu7Ge(S1-xSex)5I and Cu7(Ge1-xSix)S5I crystals reveal a behaviour typical for solid solutions and are related to the effect of temperature-related, static structural, and compositional disordering. The contributions of the temperature-related, static structural, and compositional disordering into the Urbach energy are determined. Thin amorphous films prepared on the base of Cu7GeS5I compound are shown to be characterised by a high value of the electrical conductivity which can be used for the creation of miniature solid electrolyte batteries and supercapacitors of new generation. In Cu7GeS5I thin films as well as in the single crystals, the Urbach absorption edge is formed by electron-phonon interaction which increases at the transition from the three-dimensional bulk structure to the two-dimensional planar structure. An essential characteristic of the absorption edge spectra of the thin film under investigation is a lengthy Urbach tail which results in the Urbach energy being more than three time higher than that in the crystal. The transition from the crystalline three-dimensional Cu7GeS5I superionic conductors to the two-dimensional amorphous thin films is characterized by a decrease of the electrical conductivity and the optical pseudogap, an increase of the Urbach energy, an enhancement of the electron-phonon interaction as well as an increasing structural disordering.